Method 1: Using springs to adjust your velocity

Changing springs can either increase or decrease your shooting velocity. Check out the Where to Buy Parts section for info on where to buy a Sterling spring kit.

To increase velocity, use a heavier/stronger main spring. If you wish to increase velocity even higher, use a lighter/weaker valve spring as well. To reduce velocity, simply do the opposite: use a weaker main spring (and a stronger valve spring if necessary).

The Sterling spring kit includes six springs (three main springs and three valve springs). Swapping out a spring for the next stronger or weaker spring results in an approx. 20 fps change in velocity. Don't change springs if you're trying to fine tune your shooting velocity by five or ten fps. I recommend using a dialable bolt or a regulator for fine tuning velocity.

Okay, I'm not sure if my explanation below is 100% correct...

Changing the springs affect how long the valve is open and, to a lesser extent, how far the exhaust valve is pushed in when the hammer strikes the valve. A stronger main spring (or a weaker valve spring) keeps the valve open longer, while a weaker main spring (or stronger valve spring) has an opposite effect.

Side note:
Cutting your springs will do the same (to a lesser extent). Keep in mind that cutting a spring is irreversible. Cutting the main spring decreases velocity, and cutting the valve spring increases velocity. Don't cut the springs too short, or they will not be long enough to perform or stay in position. They'll be useless. I don't recommend cutting, but now you know how it works.

How to change the main spring:

Step 1: Disconnect the pump rod from the bolt.

Unscrew the barrel	Two ways you can do this:  Unscrew the barrel and rotate the pump assembly to slip the pump rod out of the bolt;  Or just simply pull the pump rod out of the bolt and twist the pump rod 90 degrees.
	ALTERNATIVE:
And rotate the pump assembly	Or, just pull out and twist the pump rod

Step 2: Unscrew the front field strip thumb screw.

What you see if you removed your barrel in step 1.	What you see if you didn't unscrew your barrel in step 1.

Remove the front screw while holding the pump in place (or else the main / hammer spring may pop out). Tilt the gun forward and pull the pump assembly forward away from the gun to slide the main spring out. Change your main spring and/or your washers. Reverse the process to reassemble your gun.

Please note that if you wish to put washers in your hammer to adjust your shooting velocity, you'll need to unscrew the rear field strip thumb screw and detach the grip frame in order to free the hammer (because the hammer sear is blocked by the spacer bar and keeps the hammer from sliding out of the receiver). Now check your muzzle velocity with a chronograph.

How to change the valve spring:

Step 1: Unscrew the rear field strip thumb screw, remove the donkey.

Unscrew the rear screw and slide out the donkey (valve assembly body). If your bottom line hose gets in the way, don't force it or you might crimp and damage the hose. Instead, you'll need to either unscrew the front field strip thumb screw and detach the grip frame, or unscrew and detach the bottom line ASA.

Step 2: Unscrew the valve guide.

Unscrew the valve guide (a wooden dowel placed in the valve port for leverage is handy).  Remove the exhaust valve and valve spring.  Replace the spring and reverse the process to reassemble your gun. Don't forget to check your shooting velocity with a chronograph.

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